1. Field on the Invention
The present invention relates to coupled ring oscillators, for example but not exclusively to coupled ring oscillators in integrated circuits.
2. Discussion of the Related Art
Oscillators are commonly used in radio frequency communication systems and many other applications, for example, to perform mixing or frequency translation of information signals and channel selection. Oscillators are generally also present in all digital electrical systems which require a time reference, for example a clock signal, in order to synchronize operations. The ideal oscillator would be one providing a perfect time reference however typical physical practical oscillators produce signals which contain undesired noise in the form of either amplitude or phase noise.
The performance of ring oscillators, a cascaded combination of delay stages connected in a closed loop have been of great interest to circuit designers for implementation as oscillators because of their useful features such as being easily implementable, being able to achieve oscillations with low voltages, providing high frequency oscillations with low power, being electrically tuneable over a wide tuning range and being able to be tuned whilst also providing a multi-phase output. The oscillation frequency of a ring oscillator depends on the propagation delay Td per stage and the number of stages used in the ring structure M. In order to achieve self-sustaining oscillations, the ring provides a phase shift where each stage provides a phase shift of π/M for a M stage ring oscillator and an inversion provides the remaining phase shift of π. Ring oscillators have an inherent problem in that the faster a ring oscillator is to operate with a fixed delay stage the fewer the number of delay stages are required.
To overcome this problem coupled ring oscillators are known wherein two or more ring oscillators are coupled together in such a way that the system oscillates with an effect between the two oscillators. However in conventional coupled ring oscillators operating at low frequencies the compromise between phase noise or power consumption to reduce the phase noise can be problematic in oscillator design.